The most important of their constructions were their tower-temples. These were simple in plan, consisting of two or three terraces, or stages, placed one upon another so as to form a sort of rude pyramid. The material used in their construction was chiefly sun-dried brick. The edifice was sometimes protected by outer courses of burnt brick. The temple proper surmounted the upper platform. All these tower-temples have crumbled into vast mounds, with only here and there a projecting mass of masonry to distinguish them from natural hills, for which they were at first mistaken. CUNEIFORM WRITING.--We have already mentioned the fact that the Accadians, when they entered the Euphrates valley, were in possession of a system of writing. This was a simple pictorial, or hieroglyphical system, which they gradually developed into the cuneiform. In the cuneiform system, the characters, instead of being formed of unbroken lines, are composed of wedge-like marks; hence the name (from _cuneus_, a wedge). This form, according to the scholar Sayce, arose when the Accadians, having entered the low country, substituted tablets of clay for the papyrus or other similar material which they had formerly used. The characters were impressed upon the soft tablet by means of a triangular writing-instrument, which gave them their peculiar wedge-shaped form. The cuneiform mode of writing, improved and simplified by the Assyrians and the Persians, was in use about two thousand years, being employed by the nations in and near the Euphrates basin, down to the time of the conquest of the East by the Macedonians. BOOKS AND LIBRARIES.--The books of the Chaldæans were in general clay tablets, varying in length from one inch to twelve inches, and being about one inch thick. Those holding records of special importance, after having been once written over and baked, were covered with a thin coating of clay, and then the matter was written in duplicate and the tablets again baked. If the outer writing were defaced by accident or altered by design, the removal of the outer coating would at once show the true text. The tablets were carefully preserved in great public libraries. Even during the Turanian period, before the Semites had entered the land, one or more of these collections existed in each of the chief cities of Accad and Shumir. "Accad," says Sayce, "was the China of Asia. Almost every one could read and write." Erech was especially renowned for its great library, and was known as "the City of Books." [Illustration: CHALDÆAN TABLET.]